1. Field of the Invention
The present invention relates to an inkjet printing apparatus in which an ejection-orifice formation surface of a printing head including ejection orifices for ejecting ink is wiped with a wiper, and relates to a method of recovering the printing head.
2. Description of the Related Art
Printing apparatuses for printing images (including characters, symbols and the like) on printing materials (printing media) such as paper, cloth, plastic sheets and transparencies are currently used in various apparatuses. For example, such apparatuses are used in various applications, such as printing apparatuses used in printers, copiers, facsimiles and the like; or printing apparatuses used as output devices for composite electronic instruments, including computers and word processors, work stations and the like.
As techniques for printing with printing apparatuses, inkjet printing, wire dot matrix printing, thermal transfer printing, electrophotographic printing and the like are generally used. Among apparatuses using any of these techniques, apparatuses using the inkjet printing technique are configured to perform printing by ejecting ink from printing heads onto a printing medium on the basis of printing information previously created. These inkjet printing apparatuses have the following various advantages. Specifically, the apparatuses can be easily made compact; high-definition images can be printed at a high speed; printing can be performed directly on plain paper and the like, without performing special treatment; running costs are low; and noise during printing is low. Furthermore, the inkjet printing apparatuses are superior in color image printing because the inkjet printing apparatuses easily perform color image printing by using many kinds of inks (e.g., color inks).
As inkjet printing heads (hereinafter simply referred to as printing heads) used in such inkjet printing apparatuses, the following inkjet printing heads, each with a different form of ejection energy generating means which ejects ink from an ejection orifice, are known. Specifically, such printing heads include those using electromechanical transducers such as piezoelectric elements, those which heat ink by applying electromagnetic waves such as laser light, and which then eject ink droplets by the heat generating effect, those which heat liquid by using electrothermal transducers such as heat elements, and the like. Among these printing heads, inkjet printing heads which eject ink as droplets by utilizing heat energy make it possible to densely arrange printing elements including ink-ejection orifices, liquid paths communicating with the ink-ejection orifices, the above-described electrothermal transducers provided to the liquid paths, and the like. Accordingly, high-resolution printing can be performed. In particular, since the recent IC technology and the micromachining technology can be utilized for printing heads which use electrothermal transducers as energy generating elements, printing heads can be easily mounted with a high density, and manufacturing costs can be made low.
An inkjet printing apparatus performs printing by ejecting ink from printing heads while a printing medium and the printing heads are moved relative to each other. According to the way in which the printing medium and the printing heads are moved relative to each other, inkjet printing apparatuses are broadly categorized as those employing serial printing, or those employing line printing. Serial printing is a method in which printing is performed while printing heads are moved in a main scanning direction transverse to the transporting direction of a printing medium. On the other hand, line printing is a method in which long printing heads having widths not less than the maximum width of an applicable printing medium are placed, and in which printing is performed while a printing medium is conveyed in a direction perpendicular to the longitudinal direction of the printing heads.
As described above, there are various printing elements and printing methods for inkjet printing apparatuses. However, in order to maintain favorable ink ejection performance in any inkjet printing apparatuses, it is necessary to always maintain ink in nozzles to be in a state suitable for ejection, and to clean portions around ejection orifices. Specifically, there are cases where ink in the vicinities of ejection orifices of printing heads dries, and then is thickened, solidified and deposited. Furthermore, there are cases where bubbles, waste and the like enter the insides of the ejection orifices (liquid paths). These are factors which cause the clogging of ejection orifices. One method of recovering (preventing, eliminating and the like) this clogging is a recovery method in which ink is forcefully expelled from ejection orifices. Recovery methods of this kind include, for example, a suction recovery method in which a sealing system is formed in an ink ejection orifice portion by using a capping member linked to a pump, and in which ink is then forcefully expelled from ejection orifices by generating, with the pump, predetermined negative suctioning force at an ejection orifice surface (a head surface). Another such method is a pressurized recovery method in which positive pressure is applied to the insides of printing heads to forcefully expel thickened ink generated in nozzles, from ejection orifices. Furthermore, the following pre-ejection is also performed as a recovery operation. Specifically, in the pre-ejection, ink not contributing to printing is ejected to the aforementioned cap or to a predetermined ink-receiving portion before and after a printing operation, or before and after each scanning operation of printing heads.
Moreover, in addition to the above-described recovery operation in which ink is ejected or expelled from printing heads, a so-called wiping recovery operation, in which cleaning is performed by wiping foreign substances adhering to the ejection-orifice formation surfaces of printing heads, is also performed as a recovery operation essential for maintaining the ejection performance of printing heads. Specifically, there are cases where foreign substances such as ink, waste, dust and paper powder adhere to the ejection-orifice formation surfaces of printing heads. There are also cases where ink adheres to the ejection-orifice formation surfaces of the printing heads after a recovery operation in which ink is expelled and ejected. When an ejection operation is performed in such a state that ink and foreign substances adhere to the ejection-orifice formation surfaces, the ejecting directions of ink droplets are deviated, and this causes deterioration in image quality. Accordingly, in inkjet printing apparatuses, a wiping recovery operation has been heretofore performed, in which ejection-orifice formation surfaces are cleaned by causing wipers of a rubber-like elastic member or the like to wipe foreign substances by rubbing the ejection-orifice formation surfaces.
However, this wiping recovery operation has the following problems. Specifically, depending on a kind or the like of employed ink, a sufficient cleaning state cannot be obtained, and the wettability of ejection-orifice formation surfaces is made uneven. These problems are significant particularly in a case where pigmented ink is used.
To be more precise, pigmented ink is more prone to thicken or solidify on ejection-orifice formation surfaces than dye ink since pigmented ink thickens or solidifies in a shorter time than dye ink, although the pigmented ink is superior in weather resistance to dye ink since the degree of fading of color materials for pigmented ink is low even when affected by light or ozone. Because of the above disadvantage, the cleaned states of ejection-orifice formation surfaces when wiped with wipers tend to be worse in a case where pigmented ink is used than those in a case where dye ink is used. In other words, even if the ejection-orifice formation surface of a printing head is rubbed with a wiper to be cleaned, ink is deposited as a thin film on the head surface, and is then solidified in a short time. Thus, there is a problem that the printing head cannot be sufficiently recovered.
Pigmented ink is formed by allowing pigment particles to self-disperse in an aqueous solution by causing the pigment particles to adsorb resin, an activator and the like to provide hydrophilicity to the pigment particles which are originally hydrophobic, or by introducing hydrophilic groups into ends of the structures of the pigment particles. For this reason, a solidified pigmented-ink substance formed by the evaporation of water in the ink on an ejection-orifice formation surface deteriorates surface properties of the ejection-orifice formation surface more than a substance formed by the solidification of dye ink in which color materials are dissolved at the molecular level. As a result, the wettability of the ejection-orifice formation surface increases in some cases. In this case, the wettability of the ejection-orifice formation surface is uneven. In particular, in so-called resin-dispersing pigmented ink in which pigments are dispersed in an aqueous solution by using resin, the resin is prone to be adsorbed by an ejection-orifice formation surface. For this reason, the resin, in addition to pigment particles, partially increases the wettability of the ejection orifice surface. Thus, unevenness in wettability is more significant. Moreover, pigment particles have particle sizes of approximately 100 nm, and are much larger than dye molecules. Accordingly, if a wiper performs a wiping operation in a state in which pigment particles exist on an ejection-orifice formation surface, the ejection-orifice formation surface is scraped by the pigment particles, and surface properties are deteriorated in some cases. This is also a factor which makes the wettability of the ejection-orifice formation surface uneven.
In a case where the wettability of an ejection orifice surface is uneven as described above, the directions of ink droplets ejected from ejection orifices are made unstable, and the accuracy of the positions in which ink droplets land on a printing medium is decreased. This results in a significant deterioration in image quality.
To solve the above-described problem, ejecting directions are stabilized also by performing a so-called water-repellent treatment on the ejection orifice surface of a printing head to cause the ejection orifice surface to repel pigmented ink. This water-repellent treatment has an effect of stabilizing ejecting directions in an early phase of use. However, in a case where spreadable ink such as pigmented ink is used, water repellency is gradually deteriorated, and thereby ejecting directions are made unstable. Moreover, if an ejection-orifice formation surface is wiped with a wiper as described previously, spreadable pigmented ink is spread on the ejection orifice surface, and this also deteriorates water repellency. For this reason, it has been difficult to maintain the effectiveness of water-repellent treatment for a long period of time.
Moreover, a proposed printing head for pigmented ink as that described in Japanese Patent Laid-open No. Hei 11-334074, is a printing head in which only portions around ejection orifices are made hydrophilic from the beginning, to make the wettability of the portions around the ejection orifices even, and to thereby stabilize the ejecting directions of ink droplets.
However, since this hydrophilicity also deteriorates with time, it is difficult to maintain favorable ejection performance for a long period of time. Currently, known treatments for providing hydrophilicity include, for example, a UV ozone treatment. However, even with such a treatment, the degree of hydrophilicity deteriorates with time, even though hydrophilicity is shown immediately after the start of use.
In view of such a change in the water repellency or the hydrophilicity of an ejection-orifice formation surface, recovery technology called wet wiping, such as that described in Japanese Patent Laid-open No. Hei 10-138502 is disclosed. In this recovery technology, solvent (hereinafter referred to as wet fluid) with a very low volatility, e.g., glycerin or polyethylene glycol, is dispensed to a wiper for wiping an ejection-orifice formation surface, and the wiper then wipes the ejection-orifice formation surface. Thus, a change in wettability is prevented. Wet fluid is dispersed to this wiper by bringing the wiper into contact with a sponge-like wet-fluid holding portion immersed in wet fluid, and then by transferring the wet fluid held in the wet-fluid holding portion to the wiper.
This wet fluid has the following three effects. The first one is an effect of dissolving thickened or solidified ink accumulated on the ejection-orifice formation surface. The second one is an effect of a lubricant generated by being interposed between the wiper and the ejection-orifice formation surface. The third one is an effect of forming a film which protects the ejection-orifice formation surface by being dispensed to the ejection-orifice formation surface by the wiper.
However, a study done by the present inventors, on a change in a state of the ejection-orifice formation surface of a printing head with the above-described wet wiping being employed, has revealed that wet wiping is less effective in a low-temperature environment, and that the state of the ejection-orifice formation surface changes from the initial state. This change decreases the accuracy with which ejected liquid droplets land, and thereby deteriorates the quality of a printed image.
A study on the behavior of wet fluid in such a low-temperature environment has revealed that the amount of wet fluid transferred to a wiper greatly varies depending on temperature environments. That is, as the temperature of the environment where wet fluid is used decreases, the amount of wet fluid transferred to the wiper decreases. Wet fluid is supposed to be originally held continuously in a printing apparatus body for the lifetime of the apparatus body. Accordingly, wet fluid having a low saturation vapor pressure in the air, i.e., wet fluid which does not evaporate quickly, is favorable. In consideration of the solubility of thickened ink and that of solidified ink, and contact properties of the ink on each component of the printing head, a polyhydric alcohol, such as glycerin or polyethylene glycol often used as an ink composition for inkjet printing apparatuses, is preferably used. Since many of these solvents generally have high molecular weight and high viscosity, the viscosity thereof greatly increases in low-temperature environments.
FIG. 13 shows a temperature-viscosity curve for glycerin as one example. The viscosity, which is approximately 800 cp at room temperature, increases to 2300 cp at 15° C., and to 7000 cp at 5° C. In short, the viscosity drastically increases as temperature decreases.
Such an increase (thickening) in the viscosity of wet fluid in a low-temperature environment decreases the amount of wet fluid transferred from a wet-fluid holding portion to the wiper. This is considered to be caused due to the following phenomenon. That is, in a case where the wiper is in contact with the wet-fluid holding portion in a state in which wet fluid is thickened, the wet fluid and the wiper do not sufficiently come in contact with each other. Another conceivable factor is that the viscous wet fluid is difficult to leave the wet-fluid holding portion to attach to the wiper when the wiper is removed from the wet-fluid holding portion.
In contrast, the following technology is also proposed. Without using the above-described treatment fluid only for wiping, a wiper is wetted with ink by ejecting liquid (ink) from ejection orifices when the wiper moves past the ejection orifices of a printing head, and then the wet wiper wipes the ejection-orifice formation surface. In order to distinguish wiping performed by ejecting ink to a wiper from the aforementioned “wet wiping” performed by ejecting wet fluid to a wiper, wiping performed by ejecting ink to a wiper is hereinafter referred to as “ink wet wiping”.
Apparatuses which perform such wet wiping include, for example, those disclosed in Japanese Patent Laid-open Nos. Sho 59-45161, Hei 07-148934 and Hei 11-342620. Specifically, Japanese Patent Laid-Open No. Sho 59-45161 discloses a technology in which, when the ejection-orifice formation surface of a printing head is wiped along the arranging direction of ejection orifices, wiping is performed while ink is ejected to a wiper from ejection orifices which are formed at the upstream side in the wiping direction, and which does not contribute to image formation. Another disclosed wet wiping is performed while ink is ejected from not only ejection orifices not contributing to image formation, but also ejection orifices used for the image formation.
Japanese Patent Laid-Open No. Hei 07-148934 discloses a technology in which stains are removed with the wet wiper after ink is ejected to a wiper to wet the wiper. Japanese Patent Laid-Open No. Hei 11-342620 discloses a technology in which stain removal performance is improved by also using ink ejection when an ejection-orifice formation surface in which a plurality of ejection orifices are formed is wiped in a direction perpendicular to the arranging direction of the ejection orifices.
However, in these ink wet wiping techniques, pre-ejection of a large amount of ink is performed in regions in addition to a region in which a wiper comes in contact with an ejection-orifice formation surface. This is a factor for excessively soiling the inside of a printing apparatus body, and for generating a large amount of mist. These techniques also have disadvantages, such as unnecessary consumption of ink.
In view of such problems, Japanese Patent Laid-open No. 2002-166560 discloses a technique in which a wiper is wetted while ink is ejected sequentially from ejection orifices (ejection orifices at the downstream side in the wiping direction) which are about to be wiped. This makes it possible to perform wiping while ink consumption is reduced, and to reduce stains made by ink in the vicinity of a wiping device, as compared to techniques disclosed in Japanese Patent Laid-Open Nos. Hei 07-148934 and Hei 11-342620.
However, Japanese Patent Laid-Open No. 2002-166560 discloses ink wet wiping performed in all wiping operations, and this reduces the number of paper sheets that a printing apparatus can handle. This technology reduces stains made by ink in the vicinities of wipers and a wiping device, as compared to those described in Japanese Patent Laid-Open No. Hei 07/148934 and Japanese Patent Application Laid-Open No. Hei 11-342620. However, stain prevention with the technique disclosed in Japanese Patent Laid-Open No. 2002-166560 is still insufficient. That is, performing ink wet wiping in all wiping operations results in portions in the vicinity of the wiping device being stained with ink. Furthermore, since ink dispensed to the wipers evaporates and thickens, there is apprehension that wiping capability may deteriorate in the next wiping. In addition, a wiping operation may be inhibited by the evaporation, thickening and the like of ink adhering to portions in the vicinity of a movable portion of the wiping device.
As described above, an inkjet printing apparatus for which wet wiping is employed has the problem that the amount of wet fluid transferred to wipers greatly varies depending on the environment where the printing apparatus is used. Specifically, the amount of wet fluid transferred from a wet-fluid holding portion to the wipers is less in a low-temperature environment than that at room-temperature because the viscosity of wet fluid increases in the low-temperature environment. This leads to a problem that favorable image quality cannot be obtained because the wet wiping, which is not as effective as the initial wet wiping, deteriorates the ink-droplets ejection performance of each ejection orifice.
Furthermore, an inkjet printer in which ink wet wiping is used as described above has the problems that wipers and a wiping device are stained, and that ink adhering thereto evaporates and thickens to deteriorate the wiping performance of the wipers and of the wiping device.